This invention relates generally to optical imaging techniques and, more particularly, to techniques for detecting defects in solar cells. Solar cells are widely used on spacecraft and in terrestrial applications for the conversion of energy in the form of sunlight to useful electrical energy. Individual solar cells are typically arrayed in large flat or curved panels to provide power output at more useful levels. Cracks in a solar cell, caused by defective materials or arising during fabrication, have the potential to severely limit the power output of the solar panel that contains such a defective cell. Once a crack has begun, it is highly probable that it will propagate over time to develop into a more significant crack, because solar panels are typically subject to cyclic extremes of temperature. Therefore, it is important to detect not only large cracks and defects but also small ones.
Recently, some advanced multi-junction solar cells, including double- and triple-junction cells, have achieved substantially higher efficiencies than conventional high-efficiency silicon cells. These multi-junction cells utilize several layers of junctions to provide a better match to the solar spectrum. Prior to the present invention, no inspection method could be used for locating defects in multi-junction cells. For example, a method based on the use of a ring illuminator to inspect high efficiency silicon cells, as described in U.S. Pat. Nos. 6,236,044 B1 and 6,420,705 B2, is unable to inspect the multi-junction cells.
There is a related need in the art of making solar panels, to detect micro-cracks that sometimes result from spot-welding of interconnects to solar cells. Spot welding can reduce contamination that might be generated by soldering. Unfortunately, however, spot welding can cause micro-cracks if an electrode exerts too high pressure on the cell. Such micro-cracks are very difficult to detect because they are very small, sometimes less than 1 mm in length. A photo-acoustic method can be used but it is slow and requires water to be sprayed on each cell under inspection. Another disadvantage of the acoustic method is that it may be used only at the cell level. Practically all applications of solar cells involve panels having many cells. It will be appreciated, therefore, that there is a need for a method for detection of micro-cracks and other defects that is quick, non-invasive and may be used at the panel level as well as the cell level. The present invention meets and exceeds these requirements.